The proliferation of home theater systems and the demand for deep, rich bass sounds in car audio systems has created a demand for more compact subwoofers. One challenge faced by speaker designers is making subwoofers more compact without losing volume displacement capability. In order to produce high acoustic output at very low frequencies, a subwoofer must be capable of moving large quantities of air. This means increasing the area of the radiating surfaces, which is the part of the speaker that actually pushes the air. All other things being equal, increased radiating surface area translates to an increased acoustic output capability.
One technique for increasing low frequency speaker output without a significant increase in speaker enclosure size involves the use of passive radiators. Passive radiators are speakers without voice coils that are tuned to resonate at very low frequencies. When placed in a subwoofer enclosure with a speaker, the passive radiator adds to the acoustic output by resonating with the air volume inside the enclosure when the speaker produces low frequency sound.
Subwoofers that use passive radiators typically employ an enclosure with a speaker on one face of the enclosure and the passive radiator on another face of the enclosure. Another face of the enclosure is also typically used for connections and controls for an internal amplifier. The use of an internally mounted amplifier can reduce the net volume inside the enclosure. Reduced internal volume tends to reduce low frequency output capability translating to less bass. Typically, this problem is addressed by making the enclosure larger to offset volume occupied by the amplifier. It would be advantageous to have a subwoofer with increased performance capabilities without requiring an increase in enclosure size.